Sinter magnet based on fe-nd-b

ABSTRACT

A sinter magnet based on Fe-Nd-B with improved coercive field strength and reduced temperature dependency thereof consists of 25 to 50 wt. % Nd, 0.5 to 2 wt. % B, 0 to 5 wt. % Al, 0.5 to 3 wt. % O, remainder Fe and usual impurities and has an oxygen content which is adjusted by the addition of oxygen or of oxygen-containing compounds, especially of an Al and/or Nd oxide, before the dense sintering. It is obtainable by the melting together of the pure components with formation of a pre-alloy, pulverisation of the pre-alloy, alignment of the powder in a magnetic field and pressing to a green formed body, sintering at 1040° to 1100° C. and subsequent annealing at 600° to 700° C., whereby one adds the oxygen as Al or Nd oxide or via the grinding and/or sintering atmosphere.

Sinter magnets of the Fe-Nd-B type are characterised at room temperature by especially high magnetic characteristic values: Their temperature stability--mainly the coercive field strength H_(CJ) --is, however, unsatisfactory and prevents the use of the magnets in temperature-stressed machines.

Therefore, for technical uses, it is necessary to improve the magnets to such an extent that their use up to 200° C. is possible in the case of strong counterfields. In order to achieve this, especially the coercive field strength of the magnets must be further improved and the temperature dependency of the coercive field strength are reduced in order to ensure still sufficient values at comparatively high temperatures.

One has already attempted to achieve this improvement by additions of further elements to the Fe-Nd-B alloy. Thus, with additions of Dy, Tb, Al and Nb, a clear improvement of the H_(CJ) could be achieved.

Dy and Tb, as expensive, heavy rare earth metals, influence the crystal anisotropy of the Fe₁₄ Nd₂ B phase and thus also the coercive field strength in favourable way.

From M. H. Ghandehari, Appl. Phys. Lett., 48 (8) 1986, pp. 548-550, it is known that by reaction sintering of Fe₇₇ Nd₁₅ B₈ with (in comparison with the pure elements the cheaper) oxides Dy₂ O₃ and Tb₄ O₇, the increase of H_(CJ) achieved by addition of the corresponding amounts of the pure elements Dy and Tb is reduced. This allows an impairment of the positive action of the Dy or Tb addition by oxygen addition to be recognised.

Nb addition brings about separations in the Fe₁₄ Nd₂ B grains which are to act as obstacles in the case of the domain wall movement. The cause of the influence of Al on H_(CJ) has not yet been fully elucidated.

From the U.S. Pat. No. 4,588,439, it is further known that the resistance of rare earth metal-containing permanent magnets against corrosion is improved when the pre-alloy is ground in oxygen-containing atmosphere. Nothing is hereby reported regarding an improvement of the coercive field strength.

Therefore, the task forming the basis of the invention is to improve the coercive field strength in the case of sinter magnets of the type Fe-Nd-B and to reduce the temperature dependency thereof without having to add heavy rare earth metals, such as Dy and Tb.

According to the invention, this task is solved by a sinter magnet based on Fe-Nd-B which is characterised in that it consists of 25 to 50 wt. % Nd, 0.5 to 2 wt. % B, 0 to 5 wt. % Al, 0.5 to 3 wt. % O, remainder Fe and usual impurities and the oxygen content is adjusted by the addition of at least one Al and/or Nd oxide before the dense sintering.

Surprisingly, it has been shown that by introduction of oxygen in the form of Al and/or Nd oxide, there can be achieved not only a considerable increase of the coercive field strength but also a clear improvement of the temperature dependency of these properties.

Composition, production and properties of the sinter magnets according to the invention are described in more detail in the following in conjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a graphic illustration of the relationship between H_(CJ) and the Al oxide content for 4 different Fe:Nd ratios;

FIG. 2 a comparison of the H_(CJ) values for a base alloy in dependence upon the addition as Al₂ O₃ and as Al;

FIG. 3 the temperature dependency of H_(CJ) of a sinter magnet according to the invention with Al₂ O₃ addition;

FIG. 4 a graphic illustration corresponding to FIG. 1 for a base alloy and Nd₂ O₃ addition.

Sinter magnets based on Fe-Nd-B normally already contain small amounts of oxygen as impurity, depending upon the production process. Thus, the oxygen content of the Fe-Nd-B pre-alloys normally produced as intermediate products for the production of sinter magnets usually amounts to about 0.02 wt. %. By the grinding of the pre-alloys, a further increase of the oxygen content can be obtained if this is not carefully excluded by maintenance of an inert atmosphere. This oxygen enriches in the case of the later liquid phase sintering in the liquid Nd-rich phase and can lead to the formation of new phases in the case of its solidification.

The invention now depends upon the recognition that, by the precise oxygen addition in the form of an Al or Nd oxide, especially of Al₂ O₃ and/or Nd₂ O₃, these phases can be so influenced that the sought-for improvement of the properties, as explained above, is achieved.

The oxides are expediently added to the pre-alloy Fe-Nd-B before or during the grinding, preferably already in powder form. The average particle size of the added Al₂ O₃ preferably amounts to 0.5 to 0.05 μm. Nd₂ O₃ is expediently first finely ground in an attritor and then added to the alloy present for the further grinding. In this way, an especially uniform distribution of the oxide grains in the powder mixture is achieved.

In a preferred embodimental form of the invention, the sinter magnet contains 48 to 60 wt. % Fe, 38 to 50 wt. % Nd, 0.9 to 1.1 wt. % B and 0.1 to 2 wt. % Al₂ O₃. Especially preferred are hereby compositions of the mentioned type which are obtained with pre-alloys, the Nd content of which lies between 18.5 and 25 atom % and the B content amounts to 6.0 to 7.0 atom %. It is herewith possible to increase the H_(CJ), depending upon the Nd content of the pre-alloy, by 40 to 60% in comparison with the corresponding values without Al oxide addition. The increase of the coercive field strength and of its temperature stability by the Al₂ O₃ addition is thereby the more marked, the higher is the Nd content. In FIG. 1 of the accompanying drawing is graphically illustrated the dependency of the coercive field strength of 4 different Fe-Nd-B magnets upon the Al₂ O₃ content. On the lower limit of the above-given preferred range for the Nd content are achieved the best results with Al₂ O₃ additions of up to 0.8%. However, at 20 atom % Nd content, up to the upper limit of the Al₂ O₃ content of 2%, a further increase of the H_(CJ) value can be achieved.

If, to the same magnets, not Al oxide but rather only aluminium is added, then one obtains substantially smaller increases of the coercive field strength, as is shown in FIG. 2. There is illustrated graphically the dependency of the coercive field strength for the alloy Fe₇₃.5 Nd₂₀ B₆.5 upon the Al content in comparison with a magnet which was obtained from the same pre-alloy in which, however, the Al was added in the form of Al₂ O₃. The substantial improvement achieved according to the invention by Al₂ O₃ addition in comparison with Al-containing magnets is evident therefrom.

The temperature dependency of the coercive field strength H_(CJ) in the case of magnets according to the invention is substantially improved. For the special composition Fe₇₄.5 Nd₁₉.5 B₆.0 +2 wt. % Al₂ O₃, the temperature dependency is illustrated in FIG. 3.

In a further preferred embodimental form of the invention, the sinter magnet contains 2 to 6.5% Nd₂ O₃. FIG. 4 shows that, starting from a pre-alloy Fe₇₅ Nd₁₈.5 B₆, the addition of Nd₂ O₃ gives an increase of H_(CJ) in the given range of 2 to 6.5 wt. %, which amounts to up to 15%. If the Nd₂ O₃ content exceeds the given upper limit, then the non-magnetic phase portions increase.

The production of the sinter magnets according to the invention takes place by a modification of the known production methods. This consists in the melting together of the pure components with formation of a pre-alloy, pulverisation of the pre-alloy, alignment of the powder in a magnetic field and pressing of the so-aligned powder to a green formed body, sintering of the formed body at a temperature between 1040° and 1100° C. and subsequent annealing at 600° to 700° C. According to the invention, such a process is now characterised in that one uses a composition of 25 to 50 wt. % Nd, 0.5 to 2 wt. % B, 0.5 to 3 wt. % O, 0 to 5 wt. % Al, remainder Fe and usual impurities, whereby one adds at least a part of the oxygen in the form of an Al and/or Nd oxide and mixes homogeneously before the production of the green body. Preferably, the addition amounts to 0.1 to 2% Al₂ O₃ or 2 to 6.5% Nd₂ O₃. Mixtures of these oxides can also be used.

The Al and/or Nd oxide, preferably in very finely powdered form, is added, in general, to the powdered pre-alloy and ground therewith in order to achieve a distribution as homogeneous as possible. The values illustrated in the Figures were obtained with magnets produced in this way which were ground for 30 minutes, sintered for 1 hour at 1060° C. and subsequently annealed for 1 hour at 600° C. The same improvements of the magnetic properties are achieved when, alternatively, Al and/or Nd oxide is added in the case of the melting of the pre-alloy or the oxygen is added via the grinding and/or sintering atmosphere.

Four sheets of drawings attached:

FIG. 1

Dependency of the coercive field strength of Fe-Nd-B magnets upon the Al₂ O₃ content. To the different Fe-Nd-B pre-alloys ( Fe₇₄ N₁₈.5 B₆.5 ; Fe₇₃.5 Nd₂₀ B₆.5 ; Fe₇₁ Nd₂₂.5 B₆.5 ; Fe₆₈.5 Nd₂₅ B₆.5), Al₂ O₃ powder was added in the case of grinding H_(CJ) [kA/m] plotted against Al₂ O₃ content [wt. %]

FIG. 2

Dependency of the coercive field strength of Fe₇₃.5 Nd₂₀ B₆.5 magnets upon the Al content. In the case of the sample indicated with , Al was added in pure form in the case of the melting of the pre-alloy, in the case of sample indicated with , in the case of the grinding as Al₂ O₃. H_(CJ) [kA/m] plotted against Al content [wt. %]

FIG. 3

Temperature dependency of H_(CJ) of the alloy Fe₇₄.5 Nd₁₉.5 B₆.0 with 2 wt. % Al₂ O₃ addition H_(CJ) [kA/m⁻¹ ] plotted against temperature [°C.]

FIG. 4

Dependency of the coercive field strength of Fe₇₅ Nd₁₈.5 B₆.5 magnets upon the Nd₂ O₃ content. The Nd₂ O₃ powder was added in the case of grinding. H_(CJ) [kA/m] plotted against Nd₂ O₃ content [wt. %] 

We claim:
 1. An Fe-Nd-B sintered magnet having improved coercive field strength and decreased temperature dependency of the coercive field strength, said magnet consisting of the following components by weight percent: 38 to 50% Nd, 0.9 to 1.1% B, 0 to 5% Al, 0.5 to 3% O, 48-60% Fe and the remainder impurities;wherein the magnet is produced from an Fe-Nd-B pre-alloy which, after being formed, is subjected to grinding and dense sintering and wherein the content of the oxygen component is adjusted by the addition of elemental oxygen during grinding and/or sintering of the pre-alloy or by the addition of 0.1 to 2 wt. % Al₂ O₃, during formation of the pre-alloy and/or grinding of the pre-alloy.
 2. A process for the production of an Fe-Nd-B sintered magnet consisting of the following components by weight percent: 25 to 50% Nd, 0.5 to 2% B, up to 5% Al, 0.5 to 3% O, 48-60% Fe and the remainder impurities, said method comprising the steps of:(a) forming an Fe-Nd-B pre-alloy; (b) grinding the pre-alloy to a powder; (c) aligning the powder in a magnetic field; (d) pressing the powder to a green body; (e) sintering the green body at a temperature of from 1040° to 1100° C.; (f) annealing the sintered body at a temperature of from 600° to 700° C., wherein at least a portion of the content of the oxygen component is adjusted by the addition of Al₂ O₃ during formation of the pre-alloy and/or grinding of the pre-alloy.
 3. The process of claim 2 further characterized in that at least a portion of the oxygen component is adjusted by the addition of Al₂ O₃ and the oxygen component is further adjusted by the addition of elemental oxygen during the grinding of the pre-alloy and/or sintering of the green body.
 4. The process of claim 3 wherein at least a portion of the oxygen component is provided by the addition of 0.1 to 2 wt. % Al₂ O₃.
 5. The process of claim 4 wherein the Al₂ O₃ is added during step (a).
 6. The process of claim 4 wherein the Al₂ O₃ is added during step (b).
 7. The process of claim 3 further characterized in that the oxygen component is further adjusted by the addition of oxygen gas to the grinding atmosphere of step (b), or the sintering atmosphere of step (e) or to both of the grinding and sintering atmospheres.
 8. The Fe-Nd-B sintered magnet of claim 2 wherein at least a portion of the oxygen component is further adjusted by the addition of elemental oxygen during the grinding of the pre-alloy and/or sintering of the green body. 